The present invention relates to an hydraulic control arrangement for an hydromotor-driven working device, particularly mounted on an agricultural machine.
Hydraulic control arrangements of the above mentioned general type are known in the art. One such arrangement is disclosed, for example, in DE-OS No. 3,102,505. The control arrangement disclosed there has a regulating valve for proportionally controlling a pressure medium stream to a single-acting hydromotor, wherein the regulating valve for satisfying high tightness requirements secures the hydromotor by a blocking unit. A control slider influencing the pressure medium stream to the hydromotor cooperates with a preceding auxiliary slider which can be formed as a three-way stream regulator. This control arrangement has the disadvantage that the auxiliary slider is a part of a switch valve, and therefore the regulating valve can be used only in an open circuit. Thereby, this control arrangement is not suitable for a closed circuit. In addition, the control arrangement does not operate as a load-pressure-dependent system, which in certain conditions can lead to increased energy losses.
Another hydraulic control arrangement is disclosed in U.S. Pat. No. 3,592,216. This hydraulic arrangement as shown in FIG. 4 has a load-pressure-dependent operating system with two double-acting hydromotors which are supplied via a directional valve from an adjustment pump independently from one another with pressure medium. The adjusting pump is adjusted by a regulating spring in an associated adjusting device and also by the selectable maximum load pressure in direction of maximum feeding quantity. Though this control arrangement operates in advantageous manner as a load-pressure-dependent system in a closed circuit, it has the disadvantage that, for controlling of double-acting hydromotors, it operates with suitable directional valves with two motor connections and therefore is expensive. In addition, the direction valves without blocking seat valves do not satisfy high tightness requirements. For controlling a single-acting hydromotor this control arrangement does not suit well. Moreover, the regulating valves operate with different supply pressures, whereby the sensitivity for controlling changes in undesirable manner.
A further hydraulic control arrangement is disclosed in DE-OS No. 2,702,953. In this control arrangement, a guide valve is supplied from a load-pressure-dependently controlled adjusting pump with pressure medium. For excluding variations of the sensitivity of the guide valve when different adjusting pumps are used with different supply pressures, this arrangement is provided with a pressure drop valve connected between the adjusting pump and the guide valve. By exchange of the spring in the pressure drop valve, the pressure drop available at the guide valve and therefore its sensitivity can be kept constant in the event of utilization of different adjusting pumps with different supply pressures. This control arrangement operates in advantageous manner as a load-pressure-dependent system in a closed circuit. However, it has the disadvantage that it is little suitable for controlling a single-acting hydromotor which in addition must be blocked especially tightly. Moreover, the change of a spring is in many cases expensive and undesirable.